Windtunnel for calibration

[es335]

Hi.

I want to build a wind tunnel for calibration of hotwire, pitot tubes etc.

I have made the following considerations:

A large plenum where the velocity will be nearly zero.
A baffle plate at the plenum inlet.
A honeycomb, screen or a similar device inserted for to ensure a flat velocity profile and low turbulence flow (the velocity cannot be zero).
Temperature measurements in the plenum and at the exit
Velocity measurement in the plenum.
Static pressure measurement in plenum.
A smooth convergent nozzle outlet.
Maybe an exit divided in two with a convergent nozzle in each separated by a smooth tube.
Filter, moisturizer and heater at the inlet.


I highly appreciate any comments and thoughts.

 

[amorrison]

Post this also to "Aeronautic engineering other topics" group.

 

[ccfowler]

es335,

If I am reading your posting correctly, you are mainly concerned about performance of the instruments at near zero velocity and are concerned with the design of a suitable diverging section leading into the test section. On the basis of this assumption, I offer the following suggestions.

1- Skip the honeycomb, you will get better results from multiple coarse screens or perforated plates. Since the velocities will be quite low, the multiple stages of perforated plates in a gently expanding duct section will usually work better than turning vanes. Be sure to leave space between the plates to allow the flow to "adjust itself."

2- Thermal stratification and thermally induced eddy currents may be troublesome, so pay close attention to how your heaters are arranged and controlled. You may want to consider doing the heating in a "well-stirred" turbulent section well before the expanding section. Consider generous insulation of all of the ducting and expect long operating times to stabilize thermal and flow conditions. Well controlled blanket heaters to control duct wall temperatures may be very helpful.

One question: Why would you want to try to calibrate a Pitot tube at near zero velocity? The available differential pressures are so small, practical accuracy is usually not very good.

If, instead, you are wanting the near-zero velocity section just to lead into the test section, then the above comments remain valid with the following addition:

Skip the near-zero velocity section. It will be more bother than it is worth.

If you can provide more information on velocities, temperatures, expected duct sizes and lengths, etc. You can probably get more and better advice.

 

[es335]

amorrison4: Thanks. I have done that now.

ccfowler: Thank you for you comments.
My answer:
I am not concerned primarily about measuring velocity at near zero. Velocities ranging from near zero to 40m/s are my first targets, but velocities from near zero to 350m/s are needed.
And why? Well because my applications are operating in that range.

I want the plenum velocity to be near zero in order to know my upstream conditions as precise as possible. I need to calculate the velocity out from the exit hole (here the velocities are ranging from near zero and up) and this calculation I need to base on the upstream conditions. If you know of any other method, I will be pleased to hear further.

The temperature span is from approx. 20°C to 100°C.
As for the duct sizes and lengths etc., I don’t know yet. I have just started dimensioning the windtunnel. But I reckon that the outlet diameter of the nozzle will be approx. 5cm or in that area – depending on the needed outlet velocity.
The length of the duct between the plenum and outlet is not yet considered. Maybe a duct is not needed.
If you refer to the test duct dimensions I cannot give you any answer. A broad range of applications will be the target. But why is that relevant for the windtunnel?